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REFRACTORIES                                         503
Refractories for the glass industry are made, as a rule, from a mixture of calcined c a>-, consisting partly of crushed old pot material, carefully cleaned, and plastic bond clay. 1 he proportion of calcined (grog) to plastic clay usually is between 1 :1 to 3  2. in some glass plants no clay but the Grossalmerode material and pot shell was used; in o hers the plastic clay from Missouri was introduced, and, again, in some cases a smaller amount of raw flint clay was added. Some of these mixtures are as follows:
Grossalmerode clay, raw.	PER CENT 47 0	PER CENT 90	PER CENT o/i	PEK CENT OK
GroHHiilm erode clay, burnt ..... Missouri plastic clay, raw. .		10 9Q	16	90
Missouri clay, burnt .......	26 5		1 5	1 ^
Flint; clay, raw .............			15	10
Pot shell ...........	26.5	50	20	20
These mixtures, taken as a whole, gave satisfactory results with the usual soda-lime glasses. In general, it was not realized that different refractories are required for different types of glasses, and unfortunately no special efforts were made, excepting by one or two large, companies, to improve matters. With the elimination of < Sermuii clay more or lews confusion occurred, and the exclusive use of domestic clays was not, alwayn wiccesHful, for reasons which will be considered later. A search was made for, with the result that a number of new clays were brought on the market, and the mining operations of the well-known and reliable Missouri clays from the Si. I/mis district enlarged. Among the clays brought forward in this connection were t lie ball clays of Tennessee and Kentucky, already used to a large extent in the pottery and tile industries, the siliceous clay from Lester, Ark., the plastic firecla.VH from near Port-smouth, Ohio, and the clays from southern Illinois. The ArkaiiHsiH elay WUH found to be very similar to that from Grossalmerode, but it required to he .supplemented by a plastic, more aluminous clay, firing to a dense structure at n lower temperature, approximately 1,250C. For certain glasses satisfactory results have been obt ained wit h the use of only the Missouri washed plastic clays in the raw and burned state, employing in addition about 10 per cent of calcined flint clay. In other eases, lining some of the other clays mentioned, unsatisfactory results have been oh! ainrdt due principally to cracking of the pots around the bottom. The resistance of the clays to corrosion has usually been very satisfactory. The cracking lie iisr.ribed merely to the greater firing shrinkage of the American clays.
Fire Shrinkage. The differences in contraction of different clays are shown in the following table:
Clay	Drying hhrink-UKO,   in   l>('r-('<! tjl^d    Of dry volume 20.57 28. f>2 29.f>l 34.f>3 40. 61	Firing shrinkage, in percentage of volume in dried state, at temperatures of 				
		i,or>oc. 8.70 17.90 11.49 7.08	1,100C. 4.14 13.62 32.20 26.80 14.50	1,200C. 7.84 19.60 39.12 37.01 20.20	1,290C. 8.55 23.25 42.75 41.75 26.02	1,400C. 10.70 24.35 38.45 39.55 25.41
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Truwjwr I'iill cluy ...... ...... Kentucky I mil *Iy ............ """out iicjrn Ohio wnwhod ......